This invention relates to a circuit providing battery feed to a subscriber loop in a telephone circuit and more particularly to current limit means for adjusting the amplitude of current supplied to a subscriber loop as a function of loop resistance (i.e., loop length) and to longitudinal balance means for minimizing the effect of extraneous longitudinal AC voltages induced in the loop.
In many battery feed circuits for a telephone subscriber station, considerable power is dissipated in current limiting resistors which are employed to limit the maximum current amplitude. In particular, a foreign exchange battery feed circuit is a notorious power sink, especially on short loops. As is well known, a current limiting resistor is normally employed in each leg of the power feed circuit in order to maintain the longitudinal balance of each leg with respect to ground. The value of each of these resistors is in the order of 200 ohms so that a 200 ohm telephone set that is connected on a short loop will provide an off-hook current of 80 milliamperes from a standard -48 volt battery supply. The power dissipated in the 200 ohm battery feed resistors and given off as heat is then 2.5 watts. This is not only a waste of energy, but the heat that is generated may adversely affect associated microelectronic circuits currently employed in the telephone plant. It is desirable therefore to provide a current limit circuit which will control the amplitude of current supplied to a subscriber telephone station with a minimum of heat loss. Further, it is desirable to accomplish this result and at the same time maintain the longitudinal balance of the subscriber loop. It is also desirable to correct for any imbalance caused by the introduction of an active current loop means in only one lead of the subscriber loop and for the battery boost option in the other lead.